This invention relates to a method for preparing crosslinked, high DS starch esters that exhibit a high rate of biodegradability and improved high humidity resistance.
Processes for manufacturing starch esters including those with high degrees of substitution have been known for many years. As reported in Rayon Textile Monthly, Vol. 23, Jun., 1942, in an article by C. Burkhard and E. Degering entitled "Derivatives of Starch", starch acetates were first prepared in 1869 by heating starch with acetic anhydride and acetic anhydride containing 10 to 15 percent of acetic acid. The same article further describes the acetylation of starch in mixtures of acetic acid and acetic anhydride using various acid catalysts. In 1946, R. Treadway was issued U.S. Pat. No. 2,399,455 which describes the manufacture of starch acetates using specific ratios of acetic anhydride and acetic acid and mineral acid catalysis. However, the use of mineral acids to catalyze starch esterification generally leads to unavoidable depolymerization of the starch polymer and associated strength reduction in application. A recent patent publication to P. Cook et al., U.S. Pat. No. 5,138,006 issued Aug. 11, 1992, describes the preactivation of starch using acetic acid prior to acid catalyzed acetylation. While this technique significantly lowers reaction temperature and time, substantial molecular weight reduction still occurs and the process is expensive.
An improved process for the manufacture of starch triacetates was described in U.S. Pat. No. 3,795,670 issued on Mar. 5, 1974 to A. Mark and C. Mehltretter. In this patent, mineral acid catalysis is replaced by concentrated sodium hydroxide that serves a dual role of both preactivation and catalysis. Starch triacetates produced by this method retain up to 95% of their original molecular weight and are good film and fiber formers. However, starch triacetates (DS of 3.0) demonstrate a poor high humidity resistance and resultant loss in strength in molded structural composites such as disposable cutlery. Thus, there is a need for an improved process for the manufacture of high DS starch esters that exhibit a high rate of biodegradability and improved high humidity resistance.